This invention relates to seismic instrumentation. More particularly, it is directed to digitizing the analog seismic signals received from a velocity well logger.
In the prior art, velocity-logging instruments have been widely used in the petroleum industry primarily by being lowered into deep boreholes to obtain information regarding the elastic-wave properties of the surrounding rocks. In the early instruments, the logging sonde was a long tubular casing having at its top end a transmitter which, at preselected intervals, initiated a seismic pulse which traveled through the water in the borehole and the walls of the borehole to a receiver at the bottom end of the sonde. Instrumentation inside the sonde measured the time interval from the initiation of the seismic impulse at the transmitter to the first arrival at the receiver, and from this traveltime was inferred the seismic wave velocity in the rocks bordering the borehole.
As this equipment became more advanced technically, it became important to determine more than just the traveltime of the first seismic arrival. Instruments were designed which could record the complete train of analog seismic signals which were received at the detector. From this added information, it became evident that there were a number of different elastic waves reaching the detector, which had traveled by various paths from the transmitter, and which might provide a considerable amount of useful information regarding the geology of the rocks surrounding the borehole, if they could be properly interpreted.
In the meantime, and in the course of conventional seismic exploration operations, great strides were made in the field of seismic wave propagation studies by digitally recording the seismic signals and using computer programs to process the digitized data. In many ways, additional important geological information was revealed.